GEOLOGY OF SOIL. 21 



constitution of all rocks approaches so nearly to similarity, 

 that this may be laid down as the first principle in agricultural 

 chemistry, that there is one rock, consequently one soil. 



20. To the farmer all soil is primary. The question then 

 arises, How do rocks and soil affect vegetation 1 As a con- 

 sequence of the first proposition, it may be laid down as the 

 second principle of agricultural chemistry, rocks do not 



AFFECT the VEGETATION WHICH COVERS THEM. 



21. This is opposed to the geological doctrine of the 

 times, and may seem to be opposed to the statement in 

 section 18. The difference there stated may be thought to 

 produce corresponding effects in vegetation. This would be 

 true if rocks exerted any influence on soils, due to their 

 chemical constitution. A survey of the geographical distri- 

 bution of plants, used for food, will show that the common 

 doctrine of the chemical influence of rocks on vegetation is 

 not so well supported as to be considered an established 

 principle. It is not intended to deny that rocks do, by their 

 physical condition, affect vegetation. Unless it is shown 

 that their physical state depends upon their chemical consti- 

 tution, the second principle must be admitted as a general 

 truth. 



It has been distinctly avowed by Johnston in his " Lec- 

 tures," since the appearance of the first edition of these 

 pages, ** that where the soil forms only a surface layer of 

 considerable depth of transported materials, it may have no 

 relation whatever, either in mineralogical characters or in 

 chemical constitution, to the immediately subjacent rocks." 



This is the general disposition of soil. It is admitted by 

 the author above quoted, that, in Great Britain, in some 

 counties, and in nearly all the coal-fields, " the general char- 

 acter and capabilities of the soil have no relation whatever 

 to the rocks on which the loose materials immediately rest." 



